Conventional gas production wells have been constructed in subterranean strata that yield both hydrocarbons, such as gas, and an undesired amount of liquid, such as, for example, water or salt-water. The wells are usually lined with heavy steel pipe called "casing" which is cemented in place so that fluids cannot escape or flow along the space between the casing and the well bore wall. In some wells, large amounts of water are produced along with the gas from the onset of production. Alternatively, in other wells relatively large amounts of water can be produced as gas production is increased.
The production of excess water to the ground surface results in associated costs in both the energy to lift, or "produce," as well as the subsequent handling of the excess produced water after it has arrived at the surface. Moreover, the produced water must be disposed of after it has been brought to the ground surface. Surface handling of excess water, in addition, creates risks of environmental pollution from such incidents as broken lines, spills, overflow of tanks, and other occurrences. Many gas production fields and wells often rapidly become uneconomic to produce because of excessive water production.
Various apparatuses and methods have been proposed to overcome the problems associated with excess water production and the aforementioned problems associated with lifting, or producing, this water to the ground surface. Several approaches have emphasized more effective and efficient means to bring the produced water to the surface. Among these means are: smaller tubing sizes to improve lift; use of foaming agents; or installation of mechanical lift methods. These approaches, however, have not recognized that effective removal of liquid from gas wells can be accomplished by transferring the accumulated liquids subsurface to a water-absorbing disposal formation.
An evolving approach to the problems of excess water is to separate the excess water from the gas in the production well below the ground surface and allow the gas to migrate to the ground surface. The excess water is then conveyed downwardly through the well bore where it is discharged into a disposal formation of the subterranean strata. Such an approach has generally been referred to as an "in-situ" disposal method.
Generally, in-situ methods have required the availability of a suitable disposal formation, either below or above the production zone, with sufficient permeability to permit disposal of the excess water into the disposal formation. In addition, these methods have generally been used where the injection pressure gradients of the candidate disposal formations were either low or moderate (i.e., less than 0.5 psi per foot of depth). Practical limitations of the existing equipment and prohibitive costs associated with more expensive and complex pumping equipment have usually restricted use of these in-situ methods where a higher injection pressure gradient field has been encountered.
In an example of a conventional production apparatus of the in-situ type, a coupled rod pump is used for separating and producing oil from water in a well, while simultaneously disposing of the water into the producing formation or into a disposal formation below the producing formation. Such an apparatus is shown in U.S. Pat. No. 5,697,448. The apparatus employs three spaced packers (upper, middle, and lower). An oil pump is located between the upper and middle packers, and a water pump is located between the middle and lower packers. Produced oil and water are accumulated between the upper and middle packers. The oil is delivered through an opening into the oil pump and fills a cylinder associated with the oil pump. Produced water is allowed to drain through additional passages into the water pump cylinder where it accumulates for disposal. Selective pumping of the oil on the upstroke of the pump and the water on the downstroke of the pump is effected by a set of check valves associated with both the oil and water pumps.
The foregoing conventional in-situ apparatus does not solve all of the problems associated with excess water production. This apparatus does not address the problems associated with injecting excess water into disposal formations with excessive injection pressure gradients. Moreover, such an apparatus, if used in a gas production well, would most likely present gas/water separation efficiency problems given that the gas would first have to pass into the upper pump (oil pump) chamber instead of allowing the gas to vent to the ground surface.
In another example of an in-situ type apparatus, a formation injection tool, mounted to a bottom-hole tubing pump, carries out underground separation and down-bore in-situ transport and disposal of the undesired fluids into a disposal formation in the production well. Such an apparatus is shown in U.S. Pat. No. 5,425,416. In order to overcome the often-encountered moderate to high injection pressures of disposal zones, this apparatus requires the use of one or more sinker bars placed above the pump to provide the extra force necessary to overcome the injection pressure opposing the downward movement of the pump. In instances where shallow disposal zones are encountered, this apparatus requires that four or more sinker bars be used above the pump. This not only presents more expensive lifting equipment above the pump, but also adds to the overall complexity and cost of the pumping system.
In a further example of the in-situ approach, a dual action pumping system produces oil and water from the annulus on the upstroke of the pump, while injecting water on the downstroke, using gravity segregation. Such an apparatus is shown in U.S. Pat. No. 5,497,832. However, in order to overcome the often-encountered moderate to high injection pressures of disposal zones, this apparatus also requires the use of one or more sinker bars placed above the pump to provide the extra force necessary to overcome the injection pressure opposing the downward movement of the pump.
Thus, there is a need in the art for an apparatus and method that substantially obviates one or more of the limitations and disadvantages of conventional pumping systems. Particularly, there is a need for a system for allowing produced gas to vent or flow upwardly to the ground surface, while precluding produced water from being lifted to the surface. There is a particular need for such a system for disposal zones having moderate to high injection pressure gradients.